docs: def getRequiredAttrs(self):
          """Get the type URIs for all attributes that have been marked
          as required.
  
          @returns: A list of the type URIs for attributes that have
              been marked as required.
          @rtype: [str]
          """
          required = []
          for type_uri, attribute in self.requested_attributes.iteritems():
              if attribute.required:
                  required.append(type_uri)
  
          return required